2/07/2006

Visual attention is often thought to move across the visual scene much like a flashlight at night; we may become aware of the objects in only the part of the scene that the "attentional spotlight" illuminates, while other parts of the scene remain below the threshold of awareness. Like most metaphors, however, this one highlights some features of attention at the expense of others; as it turns out, it's the qualities of attention that this metaphor obscures which may be most interesting.

The spotlight metaphor of attention accords with our subjective experience: as we move throughout our environment, we can feel our attention "focus" on particular objects, much like an adjustable flashlight might. We know that attention has limited bandwidth, such that you need to focus on relatively small parts of large objects/concepts in order to fully comprehend them. And attention would even seem to move linearly from one place to another - as we look around a room, our eyes generally do not dart from far left to far right but follow a more meandering path. Just as one does not turn off a flashlight and then turn it on again once the direction of its beam has changed, attention feels like a continuous phenomenon in time and space.

As is often the case with introspection, however, these intuitions are not entirely accurate. While it does seem to take longer to shift attention between two distant locations than between two proximate locations, this difference appears to be related to the visual system and not attention per se; in fact, attention moves 10 degrees across the fovea as quickly as it can move 2 degrees. So attention does appear capable of "darting around," from one object to another, without linearly traversing the space between locations.

Also unlike a flashlight, attention can actually be split; by "covert orienting," we can attend to locations other than those we are actually looking. Attention is not bound solely by visual features either - when attending a location with two superimposed images (for example, reflections in glass by night), subjects appear to be capable of attending to only one of these objects at a time, and are almost completely ignorant of what happens to the unattended image.

Finally, the flashlight metaphor obscures one of the most dynamic and "intelligent" functions of attention. Dozens of experiments demonstrate that viewers unwilling process distractor stimuli; even when told to ignore certain images or words, imaging experiments show that we are unable to ignore them. However, there are conditions under which distractors are not processed, and it is these situations which highlight the kinds of mechanistisms that must be driving the elusive concept we call attention.

As reviewed in Lavie's 2005 Trends in Cognitive Sciences paper, distractors are not processed under conditions of high perceptual load. Perceptual load can be varied by increasing the number of distracting stimuli in a visual scene, or by increasing the distractor similarity to a given target; with high perceptual load, it is as if attentional bandwidth is so limited that it must focus tightly on only the task-relevant stimuli. In contrast, low perceptual load means that there is excess bandwidth; it's not hard to imagine an evolutionary selectivity for those humans that are paying attention to more objects that may be, but are not necessarily "distractors," and so the processing of task-irrelevant stimuli under these conditions seems mandatory.

In contrast, you see the opposite effect under high load on working memory or "cognitive control." When working memory is highly loaded with task-irrelevant information, distractors are processed more than under conditions of low load. This seems to suggest that some selection mechanism is dependent on cognitive control functions that are either the same as, or also shared with working memory functions. In contrast, highly loading working memory has minimal effect on visual search efficiency, and appears to specifically impair the ability to selectively process a visual scene by ignoring distractors.

Given the differential effects of memory load on distractor processing, it appears that attention is not a unitary phenomenon but at least a two-stage process. Low-level bandwidth-limited mecahanisms govern the basic visual processing of incoming information (probably on the basis of top-down guidance) and provide a kind of "buffer" of information which is available for subsequent processing; higher-level systems (such as those engaged by working memory tasks) then selectively govern what information is relayed to more frontal areas for further processing.